Because the hacker only changed the site title and description, it seems like he/she can only access your site through your account, and not from the hosting because if he/she hacked your host then he/she should be able to just change the .php .tpl files directly.
So, the hacker changed the title/description, either from the ACP and/or from the database itself.
As many already suggested, make sure no one else is admin (use phpmyadmin and do sql: SELECT * FROM 'phpbb_users' WHERE user_level = 1
Also, change the password to your db (the one that shown in config.php). This way, hopefully, the hacker cannot reaccess your db again.
Then, password protect (using .htaccess) your /admin/ directory, with different user/pass. This way, even if the hacker have userlevel admin, then he/she still cannot access ACP.
And lastly, check the board file for strange files, i.e. a PHP file that didn't do anything but just doing sql update for a specific entry.
But importantly, keep us posted of what happened, so everything will be in the clear.